There are two basic methods used to ignite combustion mixtures. Auto ignition through compression and spark ignition. Today a very large number of spark ignited (SI) engines are in use, consuming a limited fossil fuel supply. A significant environmental and economic benefit is obtained by making combustion engines more efficient. Higher thermal efficiencies for SI engines are obtained through operation with leaner fuel air mixtures and through operations at higher power densities and pressures. Unfortunately, as mixtures are leaned, they become more difficult to ignite and combust. More energetic sparks with larger surfaces are required for reliable operation, for example using multiple spark plugs per cylinder systems or rail-plug igniters. As more energetic sparks are used, their overall ignition efficiency is reduced because the higher energy levels are detrimental to the spark plug lifetime. These higher energy levels also contribute to the formation of undesirable pollutants. Therefore it would be desirable to have a spark plug capable of igniting leaner fuel air mixtures than traditional spark ignition sources.
Plasma ignition sources provide an alternative to traditional spark ignition and opens the door to more efficient, leaner and cleaner combustion resulting in associated economic and environmental benefits. Prior art methods and apparatuses describe using plasma as an ignition means for combustion engines. One method of generating plasma involves using a radio frequency (RF) source and a quarter wave coaxial cavity resonator to generate corona discharge plasma. The prior art uses a radio frequency (RF) oscillator and amplifier to generate the required RF power at a desired frequency. RF oscillators and amplifiers can be either semiconductor or electron tube based, and are well known in the art. The RF oscillator and amplifier are coupled to the quarter wave coaxial cavity resonator, which in turn develops a standing RF wave in the cavity at the frequency determined by the RF oscillator. By electrically shorting the input end of the quarter wave coaxial cavity resonator and leaving the other end electrically open, the RF energy is resonantly stepped-up in the cavity to produce a corona discharge plasma at the open end of the quarter wave coaxial cavity resonator. The corona discharge plasma can function generally as an ignition means for combustible materials and specifically in a combustion chamber of a combustion engine.
A quarter wave coaxial cavity resonator is designed to have an electrical length that is approximately one-quarter of the radio frequency delivered from the RF oscillator and amplifier, although cavities that are multiples of one-quarter of the radio frequency will also work. The electrical length of the quarter wave coaxial cavity resonator depends upon the physical geometry of the cavity, the temperature, pressure and environment at the open end of the cavity, as well was whether one or more dielectrics are used to plug or seal the end of the cavity.
Energy consumption is minimized and the corona discharge is maximized when the quarter wave coaxial cavity resonator and radio frequency are appropriately matched. However, the cavity still generates a corona discharge plasma for a range of frequencies around the optimal frequency as well as at higher harmonics of the optimal frequency. An unmatched quarter wave coaxial cavity resonator generally results in lower efficiencies and less power being delivered to the quarter wave coaxial cavity resonator and therefore potentially less corona discharge plasma. When the corona discharge plasma is used as an ignition source for a combustion chamber, a reduction in the amount or strength of the corona discharge plasma is undesirable as it could result in non-ignition of combustible materials in the combustion chamber. Therefore, it is best to closely match the generated radio frequency to the quarter wave coaxial cavity resonator to maximize energy efficiency and maximize corona discharge plasma generation.
However, in practice, the resonant frequency of a quarter wave coaxial cavity resonator may not be optimally matched with the RF oscillator and amplifier. This can occur for any number of reasons, including improper selection of frequency in the RF oscillator, mechanical fatigue and wearing of the quarter wave coaxial cavity resonator or dielectric, or even transient changes in the resonant frequency of the quarter wave coaxial cavity resonator due to, for example, the formation of the corona discharge plasma itself or other changes in the environment near the region of the cavity. Therefore, it is desired that the RF oscillation be dynamically generated and modulated in such a way that it is closer to the resonant frequency of the quarter wave coaxial cavity resonator in order to attain the optimal frequency for corona discharge plasma generation.
Also, the prior art systems and apparatuses that describe systems, devices, and methods for using plasma as an ignition means in a combustion engine generally require redesign of electronic ignition control systems, the fuel injection systems, or even the combustion chambers of the engines themselves to function. Therefore, there exists a need for a corona discharge plasma ignition device that can function as a replacement for a spark plug in an internal combustion engine without requiring substantial modification to the engine, ignition control system or associated connections and circuitry.
Further, to use a plasma ignition device in place of a spark plug may require that the device be approximately the same size and geometry as an existing spark plug. Because most spark plugs are small, just several centimeters, the plasma ignition device must use very high frequencies to develop standing waves in a similarly sized package. Higher frequencies generally require more expensive power supplies and conduction losses generally increase as frequencies increase. Losses require more robust power supplies, further increasing expenses and make the plasma ignition device more prone to failures due to insufficient power. Therefore there exists a need for a plasma ignition device that uses energy more efficiently or uses lower frequencies.